1. Field of the Invention
The starting point of the invention is a method for the polarimetric evaluation of a polarization-modulated light signal according to the precharacterizing clause of patent claim 1.
2. Discussion of Background
With the precharacterizing clause of patent claim 1, the invention refers to a prior art as is disclosed by Roland Stierlin, Faseroptische Sensoren [Fiber-optic Sensors], Bulletin SEV/VSE 82 (1991), pp. 21-29. In the fiber-optic current sensor described there in conjunction with FIG. 6, the light beam is split into 2 equal parts after passage through a twisted low-birefringent sensor optical fiber. The partial beams each pass through a polarizer onto one photodiode each. The two polarizers are oriented orthogonally to each other, so that without current an equal amount of light goes through both polarizers. If a current flows, the light intensity in the two partial beams changes. A normalized modulation signal is calculated from the ratio of difference and sum of the two photodiode signals.
The twisting of the sensor coil leads to the suppression of the bending-induced linear birefringence. This leads to a temperature-dependent circular birefringence and thus to a temperature-dependent position of the plane of polarization. However, for normal polarimetric detection, the average plane of polarization should be at less than 45.degree. to the two orthogonal polarization filters and detectors. This can be carried out by means of a mechanical adjustment of the polarization unit. In a high-voltage environment, this solution is hardly realizable, because of insulation problems.
U.S. Pat. No. 3,700,334 discloses a 2-beam interferometer polarimeter for measuring the intensity and polarization condition of a light signal. There, the light is subdivided by means of a beam-splitter into 2 partial beams, each of which passes through a polarization filter. The light beams emerging from the polarization filters are sent through a variable delay element, a mixing element and an analyzer, before they are recorded in a sensor unit and evaluated.